In building structures, it is often aesthetically desirable to cover large portions of the outside of the structures with as much glass and as little outside framing elements as possible, thereby providing the structures with a smooth and unbroken outside surface appearance. Therefore, it is known in the art to provide a structural adhesive bond between the building structure and the inside surfaces of glass panels to attach the panels to the building structure, thus reducing or eliminating the need for permanent outside retention members. Such bonding configurations are commonly known as "Structural Silicone Glazing" or "SSG" systems.
Typical SSG systems fall into two major classes: two-sided and four-sided. Four-sided SSG systems typically include a plurality of vertical structural mullions in combination with a plurality of horizontal structural mullions, which combine to form a mullion framework having a plurality of panel-shaped openings which are slightly smaller than the glass panels to be supported. The mullion framework is fixed about the exterior of a building structure. Each glass panel is positioned adjacent to the exterior surface of the mullion framework and over a corresponding panel-shaped opening by a plurality of temporary retaining clips, such that the edges of the panel slightly overlap the panel-shaped opening and a small gap exists between the inside surfaces of the glass panel and the framework. Structural adhesive, typically structural silicone, is then applied into the gap. After the silicone adhesive cures, it provides a structural bond between the mullion framework and the glass panel which can completely support the glass panel without any aid from the temporary retaining clips or other outside retention means. For weatherproofing purposes, additional silicone adhesive is then applied from the outside of the building into gaps created by the abutting edges of the adjacent glass panels. Disadvantageously, this "weatherbead" must be applied from the exterior of the building.
Two-sided SSG systems differ in that a structural adhesive bond as described above is provided along two (usually vertical) opposing edges of the glass panels. In two-sided SSG systems, the two edges not being structurally bonded to the mullion framework are retained by other means. This is normally done by conventional window glazing means which enclose the entire edge of the glass panels, thus not providing the smooth continuous appearance of four-sided SSG systems. As in the four-sided SSG systems, additional silicone adhesive must be applied from the outside of the building into the gap created by the abutting edges of the glass panels.
Although such SSG systems are in demand, the cost for such systems is high. As discussed above, in four-sided SSG systems, temporary mechanical retentioners for the glass panels must be installed to allow the structural silicone adhesive to cure, and then must be removed after the curing process. Sealant must then be added to cover holes left behind by the temporary fasteners. As such installation and removal processes must be performed from the exterior of the building structure, these processes are typically labor- and cost-intensive, as scaffolding must be installed to provide access to the exterior of the building. In both two- and four-sided systems, the weatherproofing joint must be installed from the building exterior, and the quality of the weatherproofing joint is highly dependent upon the skill of the field laborer installing the glass and applying the sealant.
Safety is also a concern associated with SSG systems, as high reliance is placed upon structural bonding. The structural adhesive is subject to rupture under certain loading conditions, such as high negative pressure on the lee side of the building during periods of strong winds. Such a rupture can cause a glass panel to fall from a building and crash to the ground, possibly causing catastrophic personal injury and property damage.
Various approaches to overcome the above deficiencies have been proposed, such as that disclosed in U.S. Pat. No. 4,650,702, wherein each pane of glass of the curtainwall system has a prebonded structural interface adhered along at least two of its edges. The structural interface is clipped onto the face of the mullion framework during installation to fasten the pane to the mullion framework. Also disclosed is a non-structural weatherseal between adjacent panels which is installed from the interior side of the curtainwall system.
U.S. Pat. No. 4,562,680 discloses a butt glazing system including a specially configured frame member with a front wall forming an angle of at least 135 degrees. A semicircular channel open along its forward portion is formed at the apex of the angled front wall. A special elongate mullion has a gasket formed along its front edge. The rear of the mullion insert has a T-shaped connector portion formed thereon. The "head" of the T-shaped connector portion is wider than the opening of the front of the semicircular channel, such that the head of the connector can be inserted into the channel only by introducing it at an angle and rotating it into place. When so installed, the mullion insert is held within the semicircular channel as long as it is not permitted to rotate relative to the channel. To install a curtainwall according to this system, a first glazing panel is positioned against the end mullion and the adjacent interior mullion from inside the building. A mullion insert as described is then fastened to a chevron-shaped mullion as described, and pivoted such that the gasket portion of the insert abuts the edge of the first panel. A second panel is then positioned against the opposite side of the mullion insert and against the next adjacent mullion. The glazing procedure is then repeated progressively.
Although such systems include advantages, none have proven so successful as to attain industry acceptance. Therefore, efforts continue to solve this problem.
Thus, there is a recognized need to provide a system for structural silicone glazing wherein glazing and weatherproofing may be accomplished from the interior side of the building.
There is also a need to provide such a system which is easy to install, tolerates non-regular or non-plumb installation of glass, and provides improved safety characteristics.